Apparatus for insulating electrically conductive elements

ABSTRACT

A method and apparatus for insulating a plurality of electrically conductive elements and to produce electrical cable where the method includes heating the unformed insulative material and conductive elements, applying pressure and simultaneously removing heat with a pair of rollers at a relatively low temperature and applying pressure and heat to preselected regions only of the insulative material. The method and apparatus further allow preselected portions of the cable to be unbonded so as to facilitate access to the conductive elements by simply interrupting the application of pressure to the preselected portions. The heat, press and cool and press and bond method may also be accomplished in a step and repeat process using presses to insulative flexible circuits. The above method and apparatus provide more reliable bonding at an accelerated rate of production.

This is a division of application Ser. No. 93,949 filed Dec. 1, 1970,now U.S. Pat. No. 3,802,974.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for insulatingelements and, more particularly, to a method and apparatus forsimultaneously insulating a plurality of electrically conductiveelements to achieve a more reliable bonded flat electrical cable in amore economical manner, for facilitating exposure of the electricallyconductive elements to allow electrical contact to be made, and forbonding flexible electrical circuits.

2. Description of the Prior Art

Electrical cable is simple a plurality of electrically conductiveelements aligned parallel one another in a predetermined spacingencapsulated (except for the ends of the cable) with synthetic resinelectrical insulative material. Physically, the cable appears as a thinflexible strip of synthetic resin having small embedded metallic wires.

Efforts to make electrical cable less expensively, quicker, and morereliable have spawn many methods and apparatus exemplified by the priorart. One such method, sometimes referred to as the hot-roll method,comprises locating two grooved rollers closely adjacent one another,heating the rollers to a temperature to cause fusion of the syntheticresin insulative material used and passing a plurality of conductiveelements sandwiched between insulative strips to achieve a bondedelectrical cable. Another method, such as exemplified by U.S. Pat. No.3,082,292 to R. W. Gore, comprises moving the sandwich of electricalwire and insulator between two grooved rollers and then passing theformed but unbonded cable to an oven to achieve bonding. Yet, anothermethod as exemplified by U.S. Pat. No. 3,531,045 to L. L. Emmel et al.,commonly referred to as the chill-roll process, comprises passing thelayered electrically conductive elements and insulative material througha heating device to heat the entire unbonded cable to well over thetemperature necessary for bonding and then passing the cable through twogrooved rollers which are at a temperature below that of bonding.

Each of the prior art processes provided an unreliable bond; that is,fusion of the abutting surfaces of the insulative material could not bereliably accomplished without causing substantial difficulties withother portions of the cable. For example, some of the methods requiredheating to a substantially higher temperature than necessary to causefusion to compensate for heat loss before maximum pressure is brought tobear to cause bonding; this excessive heat frequently oxidized theelectrically conductive elements thereby making future electricalcontact difficult. Excessive heat creates the generation of excessiveamounts of gas causing gas bubbles in the cable. Still another problemrelated to internal stresses formed in the insulative material duringthe insulative material's manufacture. When heated, the areas,representing overly stressed and normally stressed regions, expand atdifferent rates causing wrinkles in the insulative cover. Removingwrinkles requires critical machine adjustments and excessive machinetension applied to the insulative material during the cable-makingprocess. Upon cutting such a cable for use, the insulative materialtends to recede from the conductive elements leaving the ends of theconductive elements undesirably exposed. A further problem withoverheating occurs after pressure has been released; if the cable isstill above fusion temperature, there is a tendency for the insulativematerial to deform to its original flat shape and unbond. One solutionto the latter problem is to use more adhesive material to retardundesirable deformation. However, this results in a cable which issubstantially thicker than need be and which concurrently increasesmaterial expenses.

Another major difficulty facing the electrical cable industry and itscustomers is quickly, economically and reliably exposing theelectrically conductive elements of a bonded cable to allow electricalcontact to be made. Present methods include grinding away the insulativematerial in order to expose the conductive elements; this frequentlycauses damage to the conductive elements since it is difficult toaccurately control the grinder. Another method includes the applicationof heat to the insulative material to allow its separation from theconductive elements; this causes deformation of the cable, oxidizes theconductive elements and enhances the chance of cable delamination.Neither of the above methods are suited for fully automatic operationsince both methods disturb the positioning of the conductive elementswhich is critical for automatic connection to another electrical elementsuch as an electrical connector having multiple electrical contactpositions. For example, a three inch wide cable may easily contain sixtyelectrically conductive wires in alignment, each wire parallel to theothers. Position tolerances are ± 0.0005 inches. Grinding heating orcutting through insulation which may be 0.005 inches is exceedinglydelicate and difficult.

Still another problem faces the flexible circuit industry. A flexiblecircuit may be defined as a specifically designed layer of electricallyconductive material bonded between two layers of insulative material.Insuring a reliable bond between the two insulative layers and achievingthis bond in a relatively short time span has always eluded solution.

SUMMARY OF THE INVENTION

A solution to all of the mentioned problems is accomplished by thepresent invention which provides a method for insulating an elementcomprising the steps of providing an element positioned between layersof insulative material; moving the element and the insulative materialrelative to a first pressure station; selectively applying pressure toform the insulative material about the element; moving the formedinsulated material and the element relative to a second pressurestation; and selectively applying pressure and sufficient heat to theinsulative material to bond the layers. The invention further includes amethod for simultaneously insulating a plurality of elements includingthe bonding of layers of insulative material about the plurality ofelements with heat and pressure and for facilitating exposure of theelements at predetermined locations wherein the improvement comprisesinterrupting the application of pressure to predetermined locations ofthe insulative material.

In addition to the methods, the invention includes the apparatus foraccomplishing the methods; that is, apparatus for manufacturing cable aswell as apparatus for manufacturing flexible electrical circuits.

It is a general aim of the present invention to provide a method andapparatus for achieving a superior bonding of insulative material aboutan interior element. Other aspects of the present invention includebonding electrically insulative material about an electricallyconductive element, the provision of a method and apparatus for reducingthe heat required to achieve necessary bonding, for reducing the stressinduced in the electrically insulative material, for eliminating theneed to finely adjust the apparatus in response to excessive stresses inthe insulative material, and for cooling the bonded material quickly soas to provide a permanent form.

A corrollary aim of the present invention is to provide a method andapparatus for increasing the speed of cable manufacture in oneembodiment and in another embodiment to increase the speed of flexiblecircuit manufacture.

Another object of the present invention is to provide a method andapparatus for forming the electrically insulative material about aplurality of electrically conductive elements and for bonding theinsulative material in preselective locations only.

A further object of the present invention is to provide a method andapparatus for facilitating the exposure of the electrically conductiveelements of a bonded cable.

Yet, a further aspect of the present invention is to provide a reliableand inexpensive manner for bonding flexible circuits.

Other objects and advantages of the invention will appear from thefollowing description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a method and an apparatus forsimultaneously insulating a plurality of aligned elements to produce acable.

FIG. 2 is an enlarged partly modified diagrammatic view of a portion ofthe embodiment shown in FIG. 1.

FIG. 3 is an enlarged elevational sectional view taken along line 3--3of FIG. 2.

FIG. 4 is an enlarged diagrammatic elevational view taken within thecircle 4--4 of FIG. 2.

FIG. 5 is an enlarged elevational sectional view taken along line 5--5of FIG. 2.

FIG. 6 is an enlarged diagrammatic elevational view taken within thecircle 6--6 of FIG. 2.

FIG. 7 is a graph illustrating the temperature of the insulativematerial of a prior art process.

FIG. 8 is a graph illustrating the temperature of the insulativematerial of another prior art process.

FIG. 9 is a graph illustrating the temperature of the insulativematerial of the present inventive method.

FIG. 10 is an enlarged diagrammatic view of a portion of the view shownin FIG. 1 illustrating the forming and bonding rollers in a disengagingposition.

FIG. 11 is an elevational sectional view of the unformed and unbondedcable taken along line 11--11 of FIG. 10.

FIG. 12 is a diagrammatic perspective view illustrating the exposure ofthe conductive elements.

FIG. 13 is a diagrammatic view of a method and an apparatus formanufacturing flexible circuits.

FIG. 14 is an elevational sectional view taken along line 14--14 of FIG.13.

FIG. 15 is an enlarged diagrammatic view of a multi-layer circuitbetween etched mold plates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is susceptible of various modifications andalternative constructions, illustrative embodiments are shown in thedrawings and will herein be described in detail. It should beunderstood, however, that it is not the intention to limit the inventionto the particular forms disclosed; but, on the contrary, the intentionis to cover all modifications, equivalents and alternative constructionsfalling within the spirit and scope of the invention as expressed in theappended claims.

The product for which the present method and apparatus are directed maybe generally described as a plurality of aligned electrically conductiveelements insulated with a suitable synthetic resin material to provide acompact, flexible cable. This electrical cable has wide usage in compactsystems; the cable, or harness as it is often referred to, acts toprovide electrical paths between such components as printed circuitboards and provides a compact and light-weight element compatible withsmall components to form a very compact system.

Referring to FIG. 1, a number of electrically conductive elements 10 arebrought together and aligned one from another by being passed aroundidler rollers or drums 12, 14, 16 and 20. It is preferable that therollers 14 and 20 have grooves or recesses located in their outersurfaces in order to provide alignment guides for the conductiveelements. The rollers are located so as to provide with drive rollers 30and 31 a constant tension on the conductive elements which enhancesalignment. Two oppositely disposed supply spools 22 and 24 are providedto supply an upper layer 26 of insulative material and a lower layer 28of insulative material, respectively, so as to sandwich the conductiveelements. The insulative materials materials are commonly referred to as"webs." The lower layer 28 passes between a drive roller 23 and an idlerroller 25 which are in pressure contact while the upper layer 26 passesbetween a drive roller 27 and an idler roller 29 which are in pressurecontact. To insure tension on the upper and lower layers of theinsulative materials, the velocity of rotation of the drive rollers 23and 27 is somewhat less than the velocity of rotation of the driverollers 30 and 31. Tension is easily adjusted by simply altering therelationship of the rotational velocity of the drive rollers 23 and 27and the rotational velocity of the drive rollers 30 and 31. All of theelements mentioned thus far are suitably attached to a frame 32 (whichis illustrated in diagrammatic outline form using phantom lines) as arestorage spools 33, 34 and 35.

A major disadvantage of all prior art methods and apparatus has been thereliability of the bonding of the insulative layers for encapsulatingthe conductive elements. Often, for example, a cable is formed where thetwo layers of insulative material are brought together and apparentlyattached only to become delaminated upon handling. In accordance withone of the important aspects of the present invention, a cable is formedin which the two layers of insulative material are completely bonded soas to entrap the conductive elements to provide a flexible cable whichis permanently formed. Referring once again to FIG. 1, the alignedconductive elements 10 are passed through a heater 36 which is connectedto the frame 32. The conductive elements having the upper layer 26 ofinsulative material and the lower layer 28 of insulative material inalignment then move through a second heating station by being passedthrough a heater 38 which is attached to the frame 32. Each of theheaters are at a temperature to warm the conductive elements and theinsulative material so as to increase the plasticity of the insulativematerial; nevertheless, the temperature is below that which would causethe insulative material to bond. During this time the plurality ofconductive elements are in a parallel alignment relative one another aswell as in planar alignment and are aligned relative the two layers ofinsulative material. The heaters 36 and 38 may comprise planar infra-redradiant heaters.

It is reinterated that the deficiencies in the prior art generallycenter about the supply of heat to the cable and proper timing of theapplication of pressure. Thus, in the hot-roll process, either the heatis insufficient to cause bonding because forming and bonding wereperformed simultaneously or too much heat was supplied so that the cabledeparted the location of pressure application excessively heated therebyallowing the deformation of the cable to its original condition. Withother processes, excessive heat application caused excessive generationof gas which frequently formed undesirable gas pockets within the bondedcable. Further, the excessive application of heat causes the conductiveelements to form an oxidation coating which requires removal before areliable electrical connection can be made between the cable and anotherpart of the system in which it is employed. Still another problem isthat the insulative material is frequently received with high stressareas formed during its own manufacture. When heated, the high stresregions have a different rate of expansion than adjacent regions whichcauses excessive wrinkles at locations along the insulative material.Thus, it becomes necessary to critically tune the apparatus forming thecable by aligning to a high degree of precision the shafts for thesupply spools and the rollers and increasing the tension on thepreformed cable. When the cable is formed and severed for application,the increased tension used during manufacturing causes the insulativematerial to constrict once the tension is relieved so as to expose,undesirably, the conductive elements at the point of severance. Wrinklesalso leave air pockets and folds in the final cable causing the cable tobe rejected. Finally, the above difficulties require the speed ofprocessing the cable through the apparatus to be relatively slow,thereby increasing manufacturing time and expense.

In accordance with an important aim of the present invention, a methodand apparatus is provided which reduces the amount of heat necessary tocause bonding, reduces stress in the insulative material, eliminates theneed for fine adjustment of the machine, increases the speed ofproduction and substantially increases the reliability of the bondachieved.

To ensure proper and reliable bonding, the unformed cable 39 of thepresent invention is moved through a forming station 40 and a bondingstation 42. The forming station 40 includes two adjacent rollers 44 and46, or nips as they are frequently referred to, each connected to a heatremoval and control unit 45. The bonding station 42 includes twoadjacent rollers 48 and 50 each connected to a heat source and controlunit 49.

Referring to FIGS. 3 and 5, the outer surfaces of the rollers are shownin more detail. For example, the roller 44 includes an outer surfacehaving alternate ridges, such as ridges 64, 66 and 68, and recesses suchas recesses 70 and 72; in a like manner, the roller 46 has an outersurface having ridges 74, 76 and 78 with alternate recesses 80 and 82.The roller 48 has an outer surface having alternate ridges 64a, 66a and68a and recesses 70a and 72a; the roller 50 corresponds to the roller 48with ridges 74a, 76a, 78a and recesses 80a and 82a.

To ensure a reliably bonded and permanently formed cable, the unformedcable is bonded in a two-step operation. First, the unformed cable movesbetween the rollers 44 and 46 of the forming station 40, FIGS. 2 and 4and then between the rollers 48 and 50 of the bonding station 40, FIGS.2 and 6. Returning once again to FIG. 3, the forming station rollers 44and 46 use the conductive elements, as exemplified by the conductiveelements 10a and 10b, as mandrels around which the two layers ofinsulative material 26 and 28 are cold formed. Prior to entering theforming rollers 44 and 46, the unformed cable was heated by the heater38 to a temperature to increase its plasticity but below the temperatureat which the insulative material fuses. When the unformed cable passesbetween the rollers 44 and 46, which applies pressure selectively to theregions of the insulative material between the locations of the alignedconductive elements, that is the ridges of the rollers aligned betweenthe conductive element locations, there is a stretching of theinsulative layers in the region where they pass over a conductiveelement while in the regions between the placement of the conductiveelement there is a squeezing of the insulative material causing it tocold flow about the conductive elements. It is important to note thatprior to the forming station, the unformed cable was at an elevatedtemperature to increase its plasticity; the forming rollers 44 and 46,however, are at a lower temperature so as to cool the cable before it ismoved to the bonding station. When pressure is applied by the formingrollers there is a desired cold flow of the insulative material aboutthe conductive elements causing the insulative material to form in thedesired manner; at the same time, there is a heat transfer (see FIGS. 3and 4) from the unformed cable to the rollers causing the insulativematerial to cool and stabilize in the formed configuration. Thus, uponpassing beyond the forming rollers 44 and 46, the cable which will bereferred to as formed cable 39a, FIG. 1, appears visually to be finishedbonded cable; however, as noted in FIG. 3, abutting surfaces 92 and 94of the upper and lower layers of insulative material, respectively, arestill clearly distinguishable.

It is to be understood that the heating step accomplished by the heater38 may be dispensed with when using some insulative materials which arereadily formed at room temperature; however, with the more popularinsulative materials, it is contemplated that a heating step isdesirable. The heating step not only increases the plasticity of theinsulative material, but also serves to relieve any residual stressespresent from the insulative material's manufacturing process.

In accordance with another important aspect of the present invention,the relatively cool formed cable is heated to bonding temperature atpreselected locations only so as to minimize the total amount of heattransferred to the cable. Thus, all the disadvantages in the prior artdue to excessive heating have been obviated. The bonding rollers areidentical to the forming rollers, except that the bonding rollers are ata temperature at or slightly above that needed to bond the insulativematerial. Because only the ridges of the rollers such as ridges 64a,66a, 68a, 74a, 76a, and 78a, FIG. 5, come into contact with theinsulative material (the recesses may be 0.005 to 0.010 inches from theinsulative material) the heat of bonding is essentially transmittedthrough the ridges to the insulative material in the location wherecontact is made. For purposes of illustration, the locations aredesignated 91, 93 and 95. The transfer of heat is diagrammaticallyillustrated by wavy arrows. Any gas pockets remaining after the formingstation are closed at the bonding station. The combination of heatsufficient to cause bonding and pressure assures a complete bonding ofthe two layers of insulative material so that the abutting surfaces ofthe formed cable are no longer distinguishable.

It is important, of course, that the formed cable be raised to thebonding temperature by the time maximum pressure is applied to thecable. Referring to FIG. 6, it is seen that because of the radialdimension of the rollers, there is contact between the insulativematerial and the rollers prior to the time that full presssure isapplied; full pressure is applied to the insulative material at alocation which is coincident with a line representing the shortestdistance between the centers of the rollers 48 and 50 depicted in FIG. 6by the letters "p" and the vertical arrows (see also FIG. 4). Since thethickness of the insulative material ranges from 0.002 to 0.010 inches,heating is sufficiently rapid to cause bonding when the maximum pressureis applied. Of course, it is understood that the rate of heat conductionthrough the insulative material is a function of the particular materialused and the velocity through which the cable is moved through thebonding station.

Just as important as the addition of heat is the very rapid coolingwhich is to occur after the location of maximum pressure has been passedso that the cable does not have a chance to deform while at a hightemperature. To accomplish rapid cooling, the formed cable, asmentioned, is cooled by the forming rollers 44 and 46 to a relativelylow temperature. The cable is heated by the bonding rollers only in thelocations 91, 93 and 95, FIG. 5, establishing a substantial temperaturegradient over the width of the cable causing a heat flow from the highlyheated regions to the cooler regions which are occupied by andimmediately adjacent the conductive elements. Since good electricallyconductive elements are generally also excellent heat conductiveelements, there is a rapid dissipation of the heat by conduction throughthe conductive elements as well as dissipation of heat through theimmediate environment by the usual convection process; thus, very rapidcooling is affected immediately upon leaving the high temperaturebonding rollers so that the bond and the formation of the cable ispermanent. By way of example, it is contemplated that the locations 91,93 and 95 will occupy approximately 30 percent of the width of the cableso that only 30 percent of the cable receives heat sufficient forbonding. Because of the low total amount of heat applied for bonding andbecause of the forming step, there is less insulative layer distortionand no conductive element position shifts. Hence, a wider cable may beprocessed achieving a greater production per unit of time. It is to beappreciated that the initial cold forming of the insulative materialabout the conductive elements using the conductive elements as mandrels,the cooling of the insulative material and the conductive elements priorto the bonding station, and the application of heat to limited locationsof the cable establishes a major advantage for the present method andapparatus; that is, a reduction in the amount of heat transferred to thecable with its corollary disadvantages of excessive gas, oxidation ofthe conductive elements and redeformation after removal of the bondingpressure.

To emphasize the differences between the prior art and the presentinvention, reference is made to FIGS. 7, 8 and 9 illustrating a"heating" curve for the insulative material during three manufacturingprocesses. The hot-roll process, FIG. 7, shows by the curve 200 theinsulative material is heated above the bonding temperature at the hotroller station 202 and has a very slow heat dissipation rate asexemplified by the curve portion 204. If the temperature is abovebonding after leaving the rollers, deformation is likely to occur. Thechill-roll process, FIG. 8, shows the curve 206 substantially above thebonding temperature line 208 so as to allow forming and bonding in thecool roller station 210. The disadvantages of excessive heating havealready been mentioned. The present method, FIG. 9, shows the curve 212well below the bonding temperature line 214 during forming at the coolroller station 216 and only slightly above the line 214 at the bondingstation 218. It is to be noted that in the hot and chill-roll processes,the insulative material is heated along its entire width above thebonding temperature, while in the present method, only a relativelysmall portion of the cable is heated to bonding temperature. Hence,there is a very rapid drop in temperature after passing the bondingstation 218. Therefore, not only does the concept of forming and bondingat two separate stations provide a superior result, but in more detailthe superior results are achieved by forming about the conductiveelement so as to use them as mandrels and then, cooling prior to bondingfollowed by bonding at selected locations only.

By way of example, "Teflon" (a trademark) polytetrafluorethylene (TFE)material coated with fluorinated ethylene-propylene (FEP) has been foundto be a suitable synthetic resin insulative material with the FEP havinga temperature of fusion of about 530° F. As mentioned, the material isproduced in thicknesses for the purpose of forming cable between 0.002to 0.010 inches. It is contemplated that the heater 38 will heat theunformed cable to a temperature of about 450° F to cause desiredplasticity of the TFE while the rollers 44 and 46 are maintained at atemperature of about 100° F. Thus, the formed cable will be at sometemperature close to that of the rollers 44 and 46. The bonding rollers48 and 50 are maintained at a temperature of about 550° F so as toprovide sufficient heat for fusion of the FEP coatings. The processproceeds at about 40 feet per minute. By way of comparison, the FEPcoated TFE is a milky blue color at 530° F, tan at room temperature andtransparent at 640° F. In the chill-roll process described hereinabove,it was necessary to heat the material to the transparent condition inorder to achieve a suitable bond. It is understood that TFE will notfuse to itself if its temperature is increased; thus, a coating of FEPis necessary to achieve bonding. However, if FEP alone is used as theinsulative material, heating to about 530° F will cause fusion and thusbonding when pressure is applied.

The forming station and bonding station rollers are of identical designso as to be interchangeable; each has removable sleeves to provide thedesired groove profile. The rollers may be made of aluminum withstainless steel sleeves and with a small core for the insertion of aresistance rod heater. It is to be understood that only one roller ofeach pair may be grooved, i.e. have alternating ridges and recessesdepending upon the insulative material used. Further, there may be atemperature differential between each roller of a pair; for example, itmay be necessary for only the roller 48 to be at or above bondingtemperature to cause reliable bonding. Whether or not both rollers needhave the same temperature may depend upon the velocity of the processand the insulative material used. It is to be understood further thatthe conductive elements may be round or generally rectangular in crosssection or any other convenient shape without detracting from theinvention herein.

It is contemplated that the method and apparatus described may be usedfor other than manufacturing electrical cable. For example, opticalfibre may be insulated in the same fashion or preinsulated wires may bebrought together and supported by being encased within a cable. Hence,the term insulative material may apply to electrically insulative,thermally insulative or force (impact) insulative for example.

The bonded cable 39b proceeds to a cooler 100 which may be a roller at alow temperature before proceeding to a slitter 102 which may be providedto cut the bonded cable along a direction parallel to the longitudinalaxis of the conductive elements to form whatever width cable is desired.

Referring to FIG. 2, a modified apparatus is illustrated in phantom lineincluding a conveyor belt 140 to support a cable which is weak intensile strength. The belt carries the tensile load. Such a fragilecable is one with relatively small conductive elements which may havethe insulative material formed and bonded by grooved upper rollers only.

In accordance with another important aspect of the present invention,provision is made for facilitating exposure of the conductive elementsto allow an electrical connection to be made with the cable. Asmentioned hereinabove, various methods exist for exposing the conductiveelements of a bonded cable. The present invention contemplates passingpreselected portions of the unformed cable 39 through the forming and/orbonding stations without the application of pressure and/or heat. Thisis accomplished by mounting the rollers 44, 46, 48 and 50 to links 110,112, 114 and 116, FIG. 10, respectively, which in turn are pivotallymounted about shafts 118, 120, 122 and 124, respectively. Thus, therollers are movable from the position shown in FIGS. 1 and 2 to theposition shown in FIG. 10. Movement of the rollers from one position toanother can be accomplished manually or by a motor and timing mechanism126. The way in which bonding is prevented may be accomplished by anyone of the following: pivoting the bonding rollers 48 and 50 so as toprevent actual bonding; pivoting the forming rollers 44 and 46 so as toprevent an optimum bond; or preferably operating the forming stationrollers and the bonding station rollers in sequence so that thepreselected portion is neither formed nor bonded. When working insequence, one pair of rollers is always closed to insure alignment ofthe conductive elements. Referring to FIG. 11, the insulative layers 26and 28 remain in a sandwich about the conductive elements 10a, 10b, 10c,10d and 10e without bonding and formation taking place.

Referring now to FIG. 12, exposure of the conductive elements 10a, 10b,10d and 10e is easily accomplished by cutting through the layers 26 and28 of the insulative material in that region which has not been bonded.It has been found that the unbonded regions, perhaps several inches inlength, will balloon outwardly so that cutting and removing of theinsulative material is easily accomplished without damaging ormisaligning the conductive elements. The insulative material may befolded back from the cut to form flaps 130, 132, 134 and 136 or theflaps may be suitably cut away. Another major advantage of the presentmethod and apparatus is that accomplishing the preparation of the cableto allow easy exposure of the conductive elements does not requirestopping the cable manufacturing apparatus nor in any other waydisrupting the process.

In accordance with yet another important aspect of the presentinvention, provision is made for substantially increasing the rate ofproduction of flexible electrical circuits and for providing a reliablebond between various layers of insulative material. A flexible circuitis somewhat analogous to electrical cable in that the flexible circuitgenerally comprises a substrate layer or film of insulative materialbonded or laminated to a layer or film of electrically conductivematerial. The conductive material is covered with a second layer or filmof insulative material; it is to be understood that an alternatingpattern of multiple conductive layers and insulative layers may beformed. The process of making a flexible circuit includes attaching thesubstrate layer of insulative material and the conductive layer in asuitable fashion followed by the removal, selectively, of conductivematerial to provide a preselected circuit designed. The removal processoften is accomplished by chemical etching and is well known in theindustry. After the circuit has been formed by the conductive material,the cover layer of insulative material is then attached. It is theattachment of the cover layer of insulative material which is causingproblems by requiring an excessive amount of equipment time toaccomplish and by not achieving a reliable result.

One prior art method used to bond the cover layer of insulative materialto the remainder of the circuit is to sandwich the material in a press,heat the press so as to apply the proper bonding temperature andpressure and then cool the entire press so as to allow the circuit tosolidify. After the cooled circuit is removed, the entire press mustthen be reheated for the next cycle. Presently, if the insulativematerial is FEP such a cycle may take upwards of 25 minutes.

Referring now to FIGS. 13, 14 and 15, the concept of the methoddescribed hereinabove with regard to the manufacture of electrical cableis adaptable to the manufacture of flexible circuits. The new methodwould comprise placing a layer of insulative material and a formedcircuit pattern bonded to a substrate on a conveyor belt 200 havingsmall pegs 202 for locating the flexible circuits. The conveyor belt isdriven about two drums, a brake drum 204 and a driving drum 208. Thebelt is driven in a step and repeat fashion through the various stationsof the process; that is, the belt moves from station to station withstops at each station for a predetermined period of time. First, theflexible circuit is positioned within a heater 210 for increasing theplasticity of the insulative material much in the same fashion as theheater 38 of FIG. 1 increases the plasticity of the insulative materialof the cable. Secondly, the heated flexible circuit is moved to aforming station comprising a press 212 having an upper platen 213 and alower platen 214. An etched mold plate 215 is attached to the upperplaten while an etched mold plate 216 is attached to the lower platen.The temperature of the press 212 is below that of the unformed circuit.When the press is closed, the insulative material is cold formed aboutthe conductive material while at the same time being cooled similar infashion to the function of the first set of rollers 44, 46, FIG. 1.

The formed and cooled flexible circuit is then moved to a bondingstation, which includes a bonding press 218 having an upper platen 219,upper etched mold plate 220, lower platen 221 and lower etched moldplate 222. The press is at a temperature above that of bonding of theinsulative material. The press is closed to apply the pressure and heatsufficient to cause bonding in a fashion analogous to the operation ofthe bonding rollers 48 and 50 of FIG. 1. The flexible circuit is thenmoved to a trimming station which includes still another press 223having specially formed heads 224 and 225 to trim the flexible circuitin the desired fashion. The flexible circuit which has been trimmed ismoved beyond the trimming station to allow its removal from the conveyorbelt. Hence, a rapid but reliable bond is achieved between theinsulative layers of the flexible circuit.

Referring now to FIG. 14, there is shown in more detail a cross sectionof the forming station press 212. The flexible circuit 232 is positionedon the conveyor belt 200 located by two oppositely disposed series ofpins 202, and is comprised of a substrate layer 234 to which is bonded alayer of conductive material 236. A cover layer of insulative material240 is sandwiched between the conductive material and the conveyor belt.The lower platen or ram portion 214 with the etched mold plate 216 isthen moved upwardly to compress the layered flexible circuit against theupper platen 213. At least the lower ram platen 214 is at a temperaturelower than that of the flexible circuit so that the flexible circuit iscold formed while at the same time being cooled. At the bonding stationat least the lower platen 221 is heated to a temperature slightly abovethat necessary to cause bonding.

Referring now to FIG. 15, a multilayer flexible circuit is illustratedbetween an upper mold plate 250 and a lower mold plate 252, both ofwhich are etched to have ridges such as the ridge 254 of the upper plateand ridge 256 of the lower plate. The circuit includes three layers ofinsulative material, a bottom layer 258, a middle layer 260 and an upperlayer 262. Located to each side of the middle layer 260 are two metallicelectrically conductive elements 264 and 266. As shown, there are gapsbetween the conductive elements which ideally should be enclosed with aninsulative material to prevent possible short circuiting. This isaccomplished by forming and then bonding the upper layer 262 to themiddle layer 260 at a region designated 268 while at the same timebonding the lower layer 258 to the middle layer 260 at the regiondesignated 270. The etched mold plates 250 and 252 are designed to haveproperly dimensioned and located ridges aligned with the regions 268,270. Once again, it is appreciated that the process of heating torelieve stresses, cold forming to provide cold flow of the material andto facilitate heat conduction after the bonding stage, followed byselective heating to the temperature of bonding is accomplished easilyand quickly. As with the cable method, the electrically conductivelayers 264 and 266 act as heat conducting path to cool the circuit assoon as bonding has been accomplished to lower the temperature andprevent deformation.

I claim:
 1. An apparatus for simultaneously insulating a plurality ofelectrically conductive elements with thermal bonding insulativematerial comprising:a heater adapted to receive the combination of twolayers of thermal bonding insulative material and a plurality ofelectrically conductive elements positioned therebetween, said heaterbeing adapted to heat the thermal bonding insulative material to apliable temperature below bonding temperature, to give the insulativematerial plasticity and to relieve internal stresses therein; a firstpair of rollers adapted to receive the heated combination of the twolayers of thermal bonding insulative material and the plurality ofelectrically conductive elements positioned therebetween, at least oneroller of said first pair of rollers having an outer surface formed withalternating recesses and ridges which engage and form the layers ofinsulative material about the plurality of electrically conductiveelements; heat removal and control means for cooling at least one ofsaid first pair of rollers to conduct and remove heat from and tosolidify and stabilize the layers of thermal bonding insulative materialwith the plurality of conductive elements positioned therebetween; asecond pair of rollers in tandem with said first pair of rollers, saidsecond pair of rollers being adapted to receive the layers of thermalbonding insulative material formed about the plurality of electricallyconductive elements, at least one roller of said second pair of rollershaving an outer surface formed with alternating recesses and ridgeswhich engage the layers of insulative material formed around theplurality of electrically conductive elements; heat source and controlmeans for heating at least one of said second pair of rollers to atemperature which bonds the formed layers of insulative materialstogether and around the plurality of electrically conductive elements;and frame means for mounting said adjustable heater; said heat removaland control means, said heat source and control means, and the first andsecond pairs of rollers and for supporting the thermal bondinginsulative material and the plurality of conductive elementstherebetween.
 2. An apparatus as claimed in claim 1, wherein, said atleast one roller of said second pair of rollers having said alternatingrecesses and ridges is heated and transmits bonding heat to theinsulative material through said ridges.
 3. An apparatus as claimed inclaim 1, including means for pivoting one of the rollers of said secondpair of rollers from substantially any engagement with said insulativematerial to prevent the bonding of the layers of insulative materialformed about the plurality of conductive elements.
 4. An apparatus asclaimed in claim 3, including means for pivoting one of the rollers ofsaid first pair of rollers from substantially any engagement with saidinsulative material to prevent the forming of the layers of insulativematerial about the plurality of conductive elements.
 5. An apparatus asclaimed in claim 4, including a timing mechanism for sequencing thepivoting of said one of the rollers of the first and second pair ofrollers from substantially any engagement with selected portions of saidinsulative material to produce unformed and unbonded portions when saidone of the rollers of the first and second pairs of rollers are pivotedfrom engagement and to produce formed and unbonded portions when onlysaid one of the rollers of said second pair of rollers is pivoted fromengagement and to produce formed and bonded portions when neither ofsaid one of the rollers of both said first and second pairs of rollersare pivoted from engagement, respectively.